Electrically recording paper



I 9, 1966 v F. q. PRIBBLE 3,265,531

\ ELECTRICALLY RECORDING PAPER Filed April 27. 1962 INVEN FERDINAND c. PRIB fwd/A AT1V'ORNEY United States Patent 3,265,531 ELECTRICALLY RECORDING PAPER Ferdinand C. Pribble, Burnsville Township, Dakota County, Minna, assignor to Honeywell Inc., a corporation of Delaware Filed Apr. 27, 1962, Ser. No. 190,563 3 Claims. (Cl. 117-216) The present application is a continuation-in-part of my copending application Ser. No. 48,990, filed Aug. 11, 1960 and now abandoned, and assigned to the same assignee as the present invention.

The present invention relates to electrical recording and in particular electrical recording using a. capacitive effect.

More particularly, the present invention is directed to an improved marking medium and to a method and system for electrically recording utilizing this marking medium.

Electrically recording papers have found'considerable use in various recording devices where their greater reliability and trouble free nature over the pen and ink type recorders is necessary. Pen and ink methods have numerous shortcomings such as the tendency of the pen to clog or emit excess ink. In addition, maintenance is required to insure an adequate reservoir of ink.

There are a number of features which must be met by an electrical recording medium in order for it to be commerically feasible. Among these are:

(1) Reliability.

(2) Adequate trace with good contrast.

(3) A permanent, immediate record without fixing.

(4) Inexpensive.

(5) Usable under ambient conditions of humidity and temperature.

(6) 'Have fast writing speeds.

(7) Produce a minimum of arcing and deterioration of the paper.

(8) Be non-toxic to personnel.

A number of electrolytic recording papers have been developed in the past which have satisfied some of the above requirements. However, electrolytic papers all rely on electro-chemical changes taking place during the passage of current. Further, in order for such action to take place there is a need for ionic conduction. These prior papers have generally utilized a salt which is conductive in the presence of water in addition to the material used to produce marking. This requirement, the presence of water, thus has proven to be a major drawback to these electrolytic recording papers. The fact that the papers must be at least damp during the recording operation means that the ambient condition surrounding the paper are restricted. Further, the paper itself isdiflicult to handle and must have good wet strength. Because of the dampness, the repeated movement of a marking stylus over the same area of the paper tends to cause tearing.

Electrically-recording papers using capacitive means for producing-a trace have found more recent acceptance in that these papers are essentially dry, and thus eliminate the objections found in the use of electrolytic papers.

Capacitive papers generally operate as follows: A support that is conductive in itself or which has had a conductive film applied thereto is overlaid with a marking material which is relatively nonconducting. The marking material acts as a dielectric and as a certain voltage is attained a voltage breakdown occurs. Marking is accomplished by the exposure of a dark undersurface and in some papers by the simultaneous decomposition of the marking material to contribute to the color contrast of the trace.

Patented August 9, 1966 Several capacitive papers have been developed in the past which have utilized lead compounds as the marking material with whitening agents such as barium, calcium, and titanium compounds. However, these electrically marking capacitive papers have not met all of the above requirements. In particular, arcing is pronounced and these papers tend to smoke as a result of the intense arcing. The relative cost of producing these papers is high. Toxicity of the vaporized material also presents hazard to personnel exposed to the area of operation.

Therefore, it is the object of this invention to provide an electrically recording paper which utilizes the capacitive principle and which meets all of the above recited criteria of a desirable recording paper.

I have discovered that through the use of the improved marking medium of the present invention the disadvantages inherent in electrolytic recording and prior capacitive recording papers are obviated and a method of recording and a system for recording are made available which have heretofore not been possible using the prior art marking media. The advantages made possible in- I clude essentially dry recording with its attendant advantages and rapid, smoke free, safe recording with lower applied voltages than those available in prior capacitive papers.

Accordingly, it is a further object of the present invention to provide an improved electrical recording system which is dry and records in the absence of moisture and is substantially unaffected by humidity and in which complete recording may be elfected with the blank in the same dry state as when the blank was made.

It is a still further object of the present invention to provide an improved method of capacitive recording made possible by the discovery of the marking media of the present invention.

It is a further object of the present invention to provide an electrically recording paper utilizing a capacitive effect which has as the marking compound hexa-valent molybdenum compounds.

Other and further objects of the present invention will become apparent to those skilled in the art from a study of the following specification, appended claims, and accompanying drawing, wherein:

FIGURE 1 is a cross section of one form of the invention using a paper as prepared in accordance with the invention and shows the method and system of recording thereon.

FIGURE 2 is a perspective view of a second modification of the invention using a metallic foil prepared in accordance with the invention.

The invention may be generally described in reference to FIGURE 1 which shows a cross section of a paper prepared in accordance with one method of the invention.

The paper 11 is coated with a layer of colloidal graphite 12 and subsequently is coated with a nonconductive marking compound including molybdenum trioxide 13. In operation, an electrode means 14 is supplied to the conductive colloidal graphite 12 and a stylus 15 provides the marking electrode. A power source 16 provides the necessary current. As the stylus 15 moves over the paper breakdown of the capacitive layer 13 brings about a local alteration of the marking compound 13 which produces a color change. This provides a trace. The substrate of paper 11 can, of course, be any material so long as it does not enter into the reaction. Its sole function is to provide a base for the graphite and molybdenum marking compound.

In FIGURE 2 there is shown a modification of the present invention wherein 21 is a metallic foil such as aluminum upon which a tightly adherent layer of the marking compound 22 of the invention is located. 23 is a stylus forming with power source 24 and electrode means 25 a circuit in accordance with the invention. 26

- illustrates a trace produced in accordance with the method of the invention.

When it is desired to use a marking medium in accordance with FIGURE 1, the following procedure has proved advantageous: A suitable paper, which may be paper such as is ordinarily used in pen and ink recorders, is first coated using a suspension of colloidal graphite to produce a conducting film containing approximately .25 gram of graphite per square foot. The type of paper used is not critical, but it is desirable to use relatively thin paper to allow for large square footage to be contained in a small volume. I have found that a colloidal graphite supplied under the trade name of Aqua-Dag by Acheson Colloid Company of Port Huron, Michigan, is Well suited for making the paper conductive. The quantity of colloidal graphite used is not as critical as the resultant resistivity of the film produced. Resistivity is measured in ohms per square. The resistivity measurement is made by applying parallel bar electrodes across a two inch square of paper having the graphite treatment only. I have found that a film of graphite having a resistivity equal to 600 to 800 ohms per square wor-ks particularly well. When the resistivity is higher-i.e., thinner filmthe voltage requirement is unnecessarily high. The colloidal graphite film is applied by a doctor knife or other means well known in the art and subsequently air dried or more preferably dried at elevated temperature to produce an adherent film. Elevated temperatures are useful merely in speeding up the operation, Upon completion of the drying of the graphite layer the mark- I ing compound is applied.

While graphite coated paper performs well in the invention a wide variety of conductive substrates may be advantageously used. For example, metal coated paper or metal foils have proven satisfactory in the invention. Indeed, the use of aluminum foil or aluminized paper offers advantages in that the quantity of marking compound required to color cover the conductive substrate is less than that needed to color cover a dark material such as graphite.

Various elements will, of course, enter into the determination of a choice of a conductive substrate. These include cost, color contrast desired, etc. However, the following example is applicable to producing a coating of the marking compound of the present invention on virtually any conductive substrate. As has already been noted, aluminum foil or aluminized paper have advantages in their use over a graphite coated paper. However, the procedure and composition of the example below has proved satisfactory for a coating of the marking material.

Example I The preferred marking compound consists of a suspension of molybdenum trioxide in an ethyl cellulose-carbon tetrachloride solution. This solution was prepared according to the following schedule. Two grams of ethyl cellulose were dissolved in 100 milliliters of carbon tetrachloride. To this solution was added 60 grams of molybdenum trioxide and the resulting mixture was dispersed or suspended by ball milling to produce a homogeneous dispersion of the molybdenum trioxide in the ethyl cellulose-carbon tetrachloride mixture. This mixture was then applied by .a coating rod technique onto the colloidal graphite coated paper and the resulting paper dried at a slightly elevated temperature. Heating is used only to speed up processing and does not produce any noticeable variation in the results whether used or not. The molybdenum trioxide mixture is applied so as to obtain an ultimate thickness equivalent to approximately 1.3 grams of molybdenum trioxide per square foot. The most satisfactory results are obtained when the amount of molybdenum trioxide is between 1.0 and 1.6 grams per square foot. Of course, the results may be varied quite widely dependent upon the extent of the desired trace. That is, if the amount of molybdenum trioxide is less than 1.0 gram it tends to have a poor covering ability for the base colloidal graphite film. As previously pointed out, if a metallize-d paper or foil is used one may use a substantially thinner film of the marking compound due to the lack of a need to color cover a dark substrate. If the amount is over 1.6 grams then the electrical characteristics of the film are changed excessively and the necessary voltage to produce a trace is raised beyond desirable limits. Of course, it is to be realized that the above proportions may be varied somewhat both as to concentration and as to composition. The ethyl cellu lose has proved to be a satisfactory binding material for maintaining the molybdenum in contact with the graphite surface. However, other binding agents may also be used.

It should be realizd that the addition of other electrolyte materials to the coating containing the hexavalent molybdenum is undesirable. These additional electrolyte materials tend to alter the electrical conductivity and in some instances are hygroscopic deleteriously affecting the marking layer. Likewise, the paper must be essentially dry during the recording operation.

Other compounds of molybdenum containing molybdenum in a hexavalent state have also proved useful in the invention. For example, similar results are achieved using ammonium molybdate and molybdic acid. Concentration limits in weight per square foot for these materials are calculated on the basis of the amount of molybdenum trioxide present in the compound rat-her than upon the weight of the compound per se. The general criterion which must be met for a molybdenum compound to be within the scope of the invention is that the compound must itself be non-conducting, and non-hygroscopic, the molybdenum must be present in a hexavalent state, and the compound must be essentially white or near white in color. The last requirement is of course necessary in order that a color change produced during the passage of the electric current be detectable against the background.

Paper prepared in accordance with the preferred procedure given above is dry, non-hygroscopic, and produces a uniform black-omwhite trace when used with a recording system as outlined above. The results are dependent upon voltage and upon the stylus pressure. A stylus pressure on the paper of approximately 2 grams has been found optimum. Pressure as high as 10 grams have been found usable, although considerable arcing occurs at this pressure.

The tracing is dependent to some extent upon the Volt age across the paper. The type of trace is also dependent upon the speed of marking. At low speeds a trace is continuous, although at high speeds the trace consists of a series of closely spaced dots. The speed attainable is voltage dependent to the extent indicated below. The data represent maximum speed of writing for marking media with molybdenum concentration equal to about 1.3 gramsequivalent molybdenum trioxide per square foot.

volts D.C. equals 6 inches per second. 200 volts D.C. equals 88 inches per second.

While D.C. curent has been specified above in the example, high frequency A.C. current may also be used. Ether polarity may be used on the stylus although negative polarity results in better contrast and less arcing. Arcing can be reduced to the non-visible range by a current limiting resistor.

Having thus described the invention what I desire to obtain by Letters Patent is:

1. An electrosensitive nonelectroly-tic recording sheet which comprises a conductive base having a resistivity of less than about 800 ohms per square, a film coating on said base consisting of material selected from the group consisting of molybdenum trioxide, molybdic acid, and ammonium molybdate in a nonconductive binder, said material present in said film to the extent of about 1.0 to about 1. 6 gramsequivalent of molybdenum trioxide per square foot, said binder being present to the extent of about 3% by weight of said coating based on the equivalents of molybdenum trioxide.

2. A recording sheet in accordance with claim 1 wherecoating.

3. The recording sheet in accordance with claim 1 wherein the base is a metal foil.

References Cited by the Examiner UNITED STATES PATENTS 10 ALFRED L. LEAVITT, Primary Examiner. 1n the base is a cellulos1c paper havlng a colloidal graphite IRVING L. SRAGOW, JOSEPH REBOLD Examiners.

Assistant Examiners. 

1. AN ELECTROSENSITIVE NONELECTROLYTIC RECORDING SHEET WHICH COMPRISES A CONDUCTIVE BASE HAVING A RESISTIVITY OF LESS THAN ABOUT 800 OHMS PER SQUARE, A FILM COATING ON SAID BASE CONSISTING OF MATERIAL SELECTED FROM THE GROUP CONSISTING OF MOLYBDENUM TRIOXIDE, MOLYBDIC ACID, AND AMMONIUM MOLYBDATE IN A NONCONDUCTIVE BINDER, SAID MATERIAL PRESENT IN SAID FILM TO THE EXTENT OF ABOUT 1.0 TO ABOUT 1.6 GRAMS-EQUIVALENT OF MOLYBDENUM TRIOXIDE 